Microprocessors can be characterized by a number of ratings. The ratings can include a nominal clock rate, an input clock range, and an input clock slew rate. The nominal clock rating can be conservative. A microprocessor with a conservative nominal clock rating can have a margin of higher clock rates at which the microprocessor will operate. Operating a microprocessor above the nominal clock rating is referred to as over-clocking.
Programmable clock circuits are used by motherboard manufacturers to enable a user to overclock the microprocessor. However, when the over-clocking frequency exceeds the input frequency range of the microprocessor, the microprocessor can lose track of the clock input and hang. In addition, if the frequency of the programmable clock circuit changes faster than the microprocessor can track (i.e., the slew rating of the microprocessor is exceeded), the microprocessor can lose track of the input clock and hang.
Referring to FIG. 1, a block diagram illustrating a microprocessor system 10 is shown. The system 10 consists of a motherboard 12, a microprocessor 14, a system block 16 that includes memory, inputs/outputs, and peripherals, and a clock circuit 18. The clock circuit 18 presents a clock signal CPUCLK to the microprocessor 14 and a clock signal SYSCLK to the system block 16. When over-clocking is desired, the frequency of the signal CPUCLK is increased above the nominal value for the microprocessor 14. If the frequency of the signal CPUCLK is set too high or changes too quickly, the microprocessor 14 will lose track and hang.
When the microprocessor 14 hangs, a recovery mechanism is required to restart the microprocessor system 10. One conventional method for over-clocking recovery is for the motherboard 12 to incorporate a separate watchdog timer or resistive-capacitive (RC) delay circuit 20. The circuit 20 resets the microprocessor after a preset amount of time if not disabled. The clock circuit 18 is reset to a default frequency for restarting the microprocessor 14. The default frequency of a conventional clock circuit 18 is set during fabrication or by jumpers 22 on the motherboard.